Disk drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle the information being accessed by means of read/write heads generally located on a pivoting arm which moves radially over the surface of the disk. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the disks are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside the hub or below the disks. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. However, with the decreasing size of information storage systems, other types of bearings including hydrodynamic bearings are being developed.
In these types of systems, lubricating fluid—either gas or liquid—functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub and the stationary surrounding portion of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. The reason for the popularity of the use of air, is the importance of avoiding the outgassing of contaminants into the sealed area of the head disk housing. However, air does not provide the lubricating qualities of oil. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance.
In the case of a hydrodynamic bearing employing a liquid lubricant, the lubricating fluid and its components must be sealed within the bearing to avoid loss of lubricant which results in reduced beating load capacity. If too much lubricant evaporates from the bearing, physical surfaces of the spindle and housing can contact one another, leading to increased wear and eventual failure of the bearing system. Equally seriously, loss of a seal or failure to control the fluid level within the bearing, system could cause contamination of the hard disk drive with lubricant particles and droplets as well as outgassing-related condensation.
A further difficulty with prior art designs of liquid lubrication hydrodynamic bearings is that, during operation of the spindle motor, lubricating fluid can splash onto the shaft and migrate along the shaft into the environment. To prevent this oil migration and/or splashing, a sealing shield may be provided at one end of the shaft enclosing the bearing system.
An example of a conventional hydrodynamic bearing system 100 incorporating a shield is shown in FIG. 2(b). Hydrodynamic bearing system 100 includes a shaft 112 with a bearing element 114 secured thereto. Shaft 112 is inserted into an inner cylindrical bore of bearing sleeve 116 such that a bearing gap is formed between an outer surface of the shaft with the bearing element and an inner surface of the sleeve. The bearing gap is filled with lubricating oil 118. Oil reservoir 122 is provided at the top portion of the bearing gap to accommodate excess oil. Shield 120 having cylindrical opening 132 is placed over shaft 112 and secured to step 126 of bearing sleeve 116 such that its inner surface partially contacts lubricating oil 118. Oil fill hole 128 is provided in the shield to enable injection of lubricating oil 118 into the bearing gap.
During assembly of conventional hydrodynamic bearing system 100, bearing element 114 is press-fit onto shaft 112 which is then inserted into the inner cylindrical bore of bearing sleeve 116. Shield 120 is then placed onto step 126 of the bearing sleeve such that shaft 112 protrudes through cylindrical opening 132. Shield 120 is laser welded to the bearing sleeve. Laser welding is identified by reference numeral 124. Lubricating oil 118 is next injected into the bearing gap through oil fill hole 128.
For proper functioning of the spindle motor, it is very important to inject an adequate but not excessive amount of lubricating oil through the oil fill hole. However, the above method of assembly does not allow a manufacturer to observe the level of lubricating oil inside the system and therefore to prevent an insufficiency or overflow of lubricating oil. Manufacturer's view is obstructed by the shield. If, however, the shield is secured to the bearing sleeve after the oil is filled, laser welding of the shield causes lubricating oil to overheat because of its close proximity to the shield.
Thus, there is a need in the art for a hydrodynamic bearing system design allowing a manufacturer to observe the level of lubricating oil inside the system in order to prevent an insufficiency or overflow of lubricating oil.